1. Field of the Invention
The invention relates to base station control equipment for primarily executing channel control (allotment of radio channels, etc.) in a mobile communication system, to radio base station equipment for individually forming wireless zones under this channel control, to radio terminal equipment for offering communication services to subscribers situated in any of these wireless zones, and to a mobile communication system comprising these base station control equipment, radio base station equipment and radio terminal equipment.
2. Description of the Related Art
A CDMA system essentially has a confidentiality and interference-resistibility and can flexibly adapt itself to diversified channel allocations and multimedia communication. Because a technology of transmitting power control that can solve the near-far problem has been established recently, the CDMA system has been positively applied to the mobile communication system.
In such a mobile communication system, an individual mobile station can visit both in a close range of a radio base station and in a fringe region of wireless zone formed by the base station. Therefore, the transmission characteristics of a radio transmission channel formed between the mobile station and the radio base station can vary over a broad range.
The level of transmitting power of both or either of these mobile station and/or radio base station is therefore varied appropriately in order to secure desired transmission quality and service quality.
FIG. 20 of the accompanying drawings shows a structural example of the mobile communication system to which the CDMA system is applied.
In the drawing, a base station controller 50 is connected to a public communication network 51 and also to a plurality of radio base stations 60-1 to 60-N through inter-office links 52-1 to 52-N and through a communication link 53. A mobile station 70 visits in any of wireless zones 60-1 to 60Z-N that are individually formed by these radio base stations 60-1 to 60-N.
The base station controller 50 comprises the following constituent elements:    network interfacing part 54 connected to the public communication network 54;    a plurality N of base station corresponding parts 55-1 to 55-N each having a downlink input connected individually and directly to each of N downlink outputs of the network interfacing part 54, and connected to one of the ends of communication links 52-1 to 52-N;    selective synthesizing parts 56-1 to 56-N each having an output thereof connected directly to each of N uplinks of the network interfacing part 54, and connected directly to all of uplink output of the base station corresponding parts 55-1 to 55-N; and    a controlling part 57 having communication port connected to one end of the communication link 53 and input/output ports connected respectively to the control terminals of the network interfacing part 54, the base station corresponding parts 55-1 to 55-N and the selective synthesizing parts 56-1 to 56-N.
The radio base station 60-1 comprises the following constituent elements:    an antenna 61-1;    a transmitting/receiving part 62-1 connected between a feeding point of the antenna 61-1 and the other end of the inter-office link 52-1; and    a controlling part 63-1 having an input/output port connected to the control terminal of the transmitting/receiving part 62-1 and a communication port connected to the other end of the communication link 53.
Incidentally, the construction of the radio base stations 60-2 to 60-N is the same as that of the base station controller 60-1. Therefore, suffixes “2” to “N” are put to the corresponding constituent element and their explanation and illustration will be hereinafter omitted.
In the mobile communication system having the construction described above, the controlling part 63-1 provided to the radio base station 60-1 transmits and receives predetermined control information with the controlling part 57 provided to the base station controller 50, controls the operation of the transmitting/receiving part 62-1 on the basis of the procedure of channel control executed primarily by the controlling part 57, and forms the wireless zone 60Z-1 through the transmitting/receiving part 62-1 and the antenna 61-1.
Incidentally, the operation of each part of the radio base stations 60-2 to 60-N is the same as the operation executed in the radio base station 60-1 as described above. Therefore, explanation will be omitted for the sake of simplicity.
On the other hand, the controlling part 57 in the base station controller 50 executes the channel control described above on the basis of a procedure that is determined in advance for converting a call occurring in the mobile station 70 to a completed call.
This channel control procedure and the processing, listed below, accomplished by the controlling part 57 during the channel control process can be achieved by various known technologies, and they do not constitute the gist of the invention. Therefore, their explanation will be omitted.    a processing for forming, or releasing, paths of down speech signals, ranging from the public communication network 51 to the inter-office links 52-1 to 52-N through the network interfacing part 54 and the base station corresponding parts 55-1 to 55-N;    a processing for forming, or releasing, paths of upstream speech signals, ranging from the inter-office links 52-1 to 52-N to the public communication network 51 through the base station corresponding parts 55-1 to 55-N, the selective synthesizing parts 56-1 to 56-N and the network interfacing part 54;    a processing for transmitting and receiving signaling signals with the public communication network 51 through the network interfacing part 54; and    a processing for transmitting and receiving control information relating to channel control with the mobile station 70 through the communication link 53 and the radio base stations 60-1 to 60-N.
The controlling part 63-1 to 63-N in each radio base station 60-1 to 60-N forms radio channels (inclusive of the radio channel offered for transmission of the speech signal and the radio channel allotted to the call that occurs anew and is not yet a completed call) suitable for a predetermined channel allocation through the transmitting/receiving part 62-1 to 62-N and the antenna 61-1 to 61-N.
The control information described above is transmitted through such a radio channel.
The mobile station 70 can move from the wireless zone 60Z-1 to the wireless zone 60Z-2 adjacent thereto even during the period in which the completed call occurring in the local station still exists.
In other words, the controlling part 57 provided to the base station controller 50 cooperates with these radio base stations 60-1 and 60-2 on the basis of the channel control procedure while the mobile station 70 is moving to an area where the wireless zones 60Z-1 and 60-2 overlap with each other, or visits in such an area, for example.
During such a cooperating process, the controlling part 57 applies identifiers of these radio base stations 60-1 and 60-2 and the identifier of the first radio channel already allotted to the mobile station 70 and to be used for transmitting/receiving the speech signal, to the selective synthesizing part 56-1.
The selective synthesizing part 56-1 selects two upstream speech signals that the radio base stations 60-1 and 60-2 (transmitting/receiving parts 62-1 and 62-2) identified by these identifiers have received in parallel through the first radio channel, and synthesize these upstream speech signals and apply the resulting speech signal to the network interfacing part 54.
In consequence, the speech signal having satisfactory quality can be transmitted to the originating party or the destination party, at the completed call that has occurred in the mobile station 70 which is connected to the public communication network 51 and in which the completed call occurs, under soft hand-off (diversity hand-over) executed by the mobile station 70 in cooperation with the radio base stations 60-1 and 60-2 and the base station controller 50.
Incidentally, the mobile station 70 sets the level of transmitting power of the radio channel used for the conversation in the local station to a greater value when the level of the reception wave reaching from the radio base station 60-1 that forms the wireless zone 60Z-1 corresponding to the former visit-zone of soft hand-off described above is smaller.
The transmitting/receiving part 62-1, 62-2 in each radio base station 60-1, 60-2 measures the level of the reception wave reaching the antenna 6-1, 61-2 from the mobile station 70. Each controlling part 63-1, 63-2 sets the level of transmitting power to be transmitted by the transmitting/receiving part 62-1, 62-2 to a greater level when the level of the reception wave so measured is smaller.
Even when the relative distance between the radio base stations 60-1, 60-2 and the mobile station 70 and the transmission characteristics in the wireless zones 60Z-1 and 60Z-2 vary or fluctuate, the communication service having high transmission quality can be offered with reliability to the mobile station 70.
During the process in which the mobile station 70 moves from the area where the wireless zones 60Z-1 and 60Z-2 overlap with each other to the area belonging only to the wireless zone 60Z-2 without the extinction of the completed call described above, the controlling part 57 provided to the base station controller 50 cooperates with the controlling parts 63-1 and 63-2 provided to the radio base stations 60-1 and 60-2 on the basis of the channel control procedure described already.
In such a channel control process, the controlling part 57 allots an vacant radio channel (hereinafter called the “second radio channel”) in the wireless zone 60Z-2 corresponding to the new visit-zone in place of the first radio channel allotted previously, and instructs the controlling part 63-2 provided to the radio base station 60-2 to start transmission of the second radio channel through the communication link 53.
The mobile station 70 starts transmission to the second radio channel, in place of the first radio channel at transmitting power equal to any of the following prescribed values on the basis of the channel control procedure:    a prescribed value equal to transmitting power of the first radio channel transmitted by the radio base station 60-1;    a prescribed value PT computed as a value satisfying both of the following conditions (a) and (b), and given in advance as a constant which is, for example, the base station controller 50 and the radio base station 60-2 written into an ROM, etc., mounted to the controlling part 57, 63-2;
(a) Desired minimum transmission quality Eb/lo can be secured for the radio channels relative to the field strength Ec/lo of a rated pilot channel when radio transmission is executed for all the radio channels; and
(b) The following formula is given as to the power distributions PP, PS, PPG given to a pilot channel, a synchronizer channel and a paging channel, respectively, on the basis of the channel allocation and the office establishment condition of the radio base stations 42-1 and 42-2, the number NSP of the speech channels to be used for transmission of the speech signal among the radio channels described above, an upper limit value U set in advance as means for preventing power distributed to these communication channels from becoming excessively large even when the number of subscribers is excessively small and the width X of the range of transmitting power to be varied during the process of transmitting power control:PT={(1−PP−PS−PPG)/NSP}−X/2(PT≦U)  (Eq. 1)
Consequently, the mobile station 70 can consecutively receive the communication service even when it is moving so long as it visits in any of the wireless zones 60Z-1 to 60Z-N.
In the prior art example described above, the level of transmitting power of the second radio channel at which the radio base station 60-2 forming the wireless zone 60Z-2 corresponding to the new visit-zone is to start transmission has been set to the prescribed value described above, for example, in any of the existing mobile communication systems to which the CDMA system is applied, because transmitting power is not stipulated as the specification and the rating.
However, when this prescribed value is excessively large, interference to the downlink of the radio transmission channel applied to transmission of the speech signal becomes great in any of the mobile stations other than the mobile station 70 in which the completed call occurring exists in parallel among the mobile stations located in any of the wireless zones 60Z-1 to 60Z-N. When the prescribed value is excessively small, on the contrary, speech quality possibly drops remarkably.
The prescribed value described above can be set highly accurately, in principle, as the value adapted to the channel allocation in combination with the construction and arrangement of the wireless zones.
However, transmission quality of the downstream radio transmission channels formed between the radio base station 60-1 to 60-N and the individual mobile station located in the wireless zone 60Z-1 to 60Z-N can generally fluctuate drastically and incessantly depending on the distribution of landform and planimetry in these wireless zones 60Z-1 to 60Z-N.
In other words, this prescribed value cannot be practically a common value to all the radio base stations 60-1 to 60-N. Even when a complicated processing is executed to optimize the zone configuration and the distribution of landform and planimetry, the prescribed value cannot be computed always as a value that sufficiently satisfies the requirements for maintenance and operations.
For these reasons, the prescribed value has been set to a rather greater value in the existing mobile communication systems to secure a margin necessary for securing highly accurately desired service quality without taking numerous conditions on the system configuration and on the operation and maintenance and inspection into consideration.
According to such a prescribed value, transmitting power of the individual radio channels these radio base stations 60-1 to 60-N transmit adds to the running cost of the radio base stations 60-1 to 60-N.
Therefore, when the number of mobile stations (the number of simultaneous speakers) that transmit and receive in parallel the speech signals by using in common the common band decreases according to the increase of interference described above, the running cost might further increase.